1. Field of the Invention
The present invention relates to a bridge circuit for interconnecting networks for storing information of the addresses of stations in a local area network which send and receive a message, effectively connecting two local area networks with each other.
2. Description of the Prior Art
A conventional bridge circuit for interconnecting networks with each other, is the subject of U.S. Pat. No. 4,597,078 assigned to Digital Equipment Corporation. The bridge circuit is dealing with two local networks. There is connected to a first local network a first control device and to a second local network a second control device. When a message is sent from the first local network to its associated control device, the message is examined and if it is acceptable for further transmission, the control device sends an interrupt signal to a microprocessor. Meantime the message has been stored in a RAM. In response to the interrupt signal, the microprocessor is aware of which network has sent the message and fetches the destination address of the message from the RAM to a look-up controller circuit. The look-up controller circuit is designed to take the destination address information and compares it against a group of destination addresses which are held in a look-up RAM. The destination address information, in the look-up RAM, includes a tag which indicates whether the destination address belongs to a station in the first or second local network. If there is a match between the destination address of the message and a destination address in the look-up RAM, the microprocessor will send the message to the proper station through the bridge, or ignore the message if the sending station is on the same local network as the receiving station. If there is no match, i.e., it represents a new station, the message will be sent to the other network. In the latter case when such a new station becomes a source station, its address will be recorded so that the next time such a station is a destination.
In the above-described bridge circuit, the microprocessor stores the addresses of the sending and receiving stations in the memory, creates a table of the addresses of the stations corresponding to each network, retrieves the addresses in the table, and judges whether the message is to be sent to the other network or not. As the result, the transferring speed of a message is restricted not by the physical speed of the networks but by the processing speed of the microprocessor in the bridge circuit.